Lower Jurassic-Lower Cretaceous belemnite biogeography and the development of the Mesozoic Boreal Realm

Abstract

The distribution of Lower Jurassic-Lower Cretaceous belemnites is reviewed in the light of recent taxonomic and biogeographic studies and it is seen that belemnite provinciality developed in the late Lower Jurassic, when distinct Arctic taxa are first recognised. In the Middle Jurassic, Boreal and Tethyan belemnite realms are apparent, the Boreal Realm being divisable into informal Boreal-Atlantic and Arctic provinces, which can be recognised up into the Lower Cretaceous. In the Upper Jurassic and Lower Cretaceous distinct Arctic belemnites are found in association with Tethyan ammonites in California, and it is this factor, in association with an observed increase in belemnite diversity northwards from the Boreal-Atlantic to the Arctic provinces that suggests that latitudinal temperature gradients were not the overall control influencing the development of Boreal and Tethyan belemnite realms. Instead, the hostile environment of the Boreal-Atlantic area (the European Archipelago) is seen as an effective barrier to the migration of Tethyan belemnites to the Arctic, and vice versa. However, high sea level stands may have helped blur these distinctions, as in the late Valanginian-Hauterivian, when Arctic taxa invaded Europe, and Tethyan taxa the periphery of the Arctic basin.